Recently, USB connector for headset has been requested in the new generation of portable equipments (e.g. mobile phones).
An USB connector is used both for USB digital data transfer and for USB headset audio listening. When the headset amplifier is selected for audio listening, the amplifier is powered ON, while the USB circuit is switched OFF with high impedance. The reverse happens when the USB circuit is selected for data transfer, i.e. the USB circuit is ON and headset amplifier is in power down with high output impedance.
Typically, a mobile phone with USB headset amplifiers comprises a USB charger arranged to supply a operative voltage VBUS of about 5V or 5.25V, a USB circuit and one or more headset amplifiers. The USB circuit is arranged to operate between a first reference voltage VUSB and a further referenced voltage GND. The USB circuit is provided with a first output terminal to provide a first digital data D1 and a second output terminal to provide a second digital data D2. Both the first D1 and second D2 digital data having a voltage value corresponding to the reference voltage VUSB (3V or 3.3V) or to the further reference voltage (0V). Moreover, according to USB requirements, the USB circuit has to withstand with a possible short-circuit between the high/low logical level of the transmitted digital data and the reference voltage VBUS provided by the USB charger without damaging. The headset amplifier positive supply voltage is usually from 1.5V to 2.1V, and the headset amplifier negative supply voltage is usually from −1.2V to −1.5V obtained with a negative charge pump circuit, as known by the man skilled in the art.
FIG. 3 shows the circuital implementation of an output stage 300 of a headset amplifier (not shown in the figure).
The output stage 300 comprises a PMOS transistor MP and NMOS transistor MN connected in series one another between the positive voltage supply of the amplifier VCCHS (e.g., 1.8V) and the negative voltage supply of the amplifier VSSHS (e.g. −1.4V).
The circuital implementation of FIG. 3 is used for the majority of headset amplifiers on the market, and anyway it is the only possible for the values of headset amplifier positive/negative supply voltage less than few volts (as indicated above), as happens in modern technologies for mobile phones and particularly mobile phones with USB headset amplifier.
In USB mode (digital data transmission), the USB transceiver 301 is supplied by VUSB (typically, 3V or 3.3V) and the first digital signal D1 present on the output terminal O1 of the USB transceiver 301 has a voltage value equal to 3.3V and then is higher than the headset amplifier positive voltage supply VCCHS (1.8V), forwarding the junction diode between the drain terminal D and the body terminal B of the PMOS transistor MP. In addition, the USB transceiver 301 cannot source this current without degrading the data transmission speed, but also this huge current can also lead to reliability problems or to the breakdown of the junction of the PMOS transistor MP. The situation is even worsened considering that the output terminal O1 of the USB transceiver 301 can be accidentally short-circuited to the reference voltage VBUS (5.25V) of the USB charger, as already previously explained.
A first prior art solution is to connect the body terminal B of the PMOS transistor to the reference voltage VBUS of the USB charger, accepting a certain amount of body effect degradation on its threshold voltage. However, this first solution has several drawbacks: the PMOS transistor MP has already large dimensions for driving purpose, which would further increase just to compensate the body effect; the reference voltage VBUS is usually not present in headset audio listening mode; typically, the USB charger providing the reference voltage VBUS cannot be present in the same chip where USB transceiver and USB headset amplifier resides, thus VBUS signal is not accessible.
A second prior art solution, schematically illustrated in FIG. 4, is to connect the body terminal B of the PMOS transistor MP of the output stage 400 to the headset amplifier positive voltage supply VCCHS when the headset audio listening mode is selected (digital control signal HS=1), while switching it to the reference voltage VBUS of the USB charger when the USB data transmission mode is selected (digital control signal HS=0).
This second prior solution solves the first drawback related to body effect degradation. However, it requires that the reference voltage VBUS of the USB charger is powered down (pull-down) or in high-impedance when the USB audio listening mode is selected, otherwise the junction between the source terminal S2 and the body terminal B2 of the PMOS transistor MS2 is forward biased introducing unacceptable huge current consumption from the reference voltage VBUS to the positive voltage supply VCCHS and reliability problems. More important, this second prior art solution cannot be adopted when the VBUS is not accessible, i.e. the USB charger is not in the same chip of the USB circuit and the headset amplifier.